Use of solid electrolytes goes back to Michael Faraday's report in 1834 that solid lead fluoride at red heat would conduct electricity as would the metallic vessel in which he was heating it. More recently, the use of polymers of ethylene oxide and/or propylene oxide, sometimes along with other copolymeric materials, has provided a solid polymer material useful as an electrolyte and as a positive electrode material in high rate thin film batteries or capacitors capable of pulse discharge. Such materials are described, for example, in U.S. Pat. No. 4,578,326, issued Mar. 25, 1986 to Michel Armand, et al and in U.S. Pat. No. 4,683,181, issued July 28, 1987, to Michel Armand, et al. A more general description of such electrolytes can be found in the May 20, 1985 volume of Chemical and Engineering News, pages 43, 44 and 50-57. This article, particularly on pages 54-55 discusses polymeric solid electrolytes including poly (ethylene oxide) polymers (PEO) and polymers using a highly flexible polyphosphazene backbone to which short-chain polyether groups are attached.
High energy density, rechargeable solid polymer electrolyte (SPE) using solid state batteries, for example, the Li/SPE/TiS.sub.2 or Li/SPE/V.sub.6 O.sub.13 systems, promise virtually maintenance-free reliable operation over many thousands or ten of thousands of cycles if certain physico-chemical problems can be overcome. The most important problems are as follows:
(1) The low mobility of Li.sup.+ in the SPE. PA0 (2) The difficulty of maintaining intimate contact between the SPE and the lithium negative and TiS.sub.2 interaction positive electrodes. PA0 (3) The occasional growth of a lithium dendrite that penetrates the SPE on recharging. PA0 (4) Low positive electrode utilization on rapid charging. This problem is not due to the SPE itself, but reflects a limitation of existing intercalation positive electrodes (e.g., TiS.sub.2).
(5) Long-term thermal stability at the temperatures at which SPE batteries are likely to operate (e.g., 80.degree.-100.degree. C.).
Research has expanded considerably in the development of solid polymer electrolytes for applications in high energy density batteries, specific ion sensors, and electronic displays. Wright and coworkers (British Polymer Journal 7, 319(1975) and Polymer 14, 589 (1973)) originally observed the ionic conductivity of complexes of alkali metal salts with poly(ethyleneoxide). M. B. Armand, and coworkers (Fast Ion Transport in Solids, Ed. P. Vashishita, North Holland, N.Y. (1979) p. 131; Second International Meeting on Solid Electrolytes, Saint Andrews University, Scotland (1978); Journal of the Electrochemical Society 132, 1333(1985)) developed a detailed understanding of the ionic conductivity of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) salt complexes and proposed their use as solid polymer electrolytes in high energy density batteries. For the PEO-salt complexes, it has been suggested that the alkali metal cations reside in the helical tunnel of PEO, which is in a (T.sub.2 GT.sub.2 G) conformation. This structure is similar to the complexes between Li.sup.+, Na.sup.+, K.sup.+ and crown ethers. However, PEO and PPO complexes show ionic conductivity only above 100.degree. C. Recently, Blonsky, et al. (J. Amer. Chem. Soc. 106, 6854 (1984)) synthesized poly(phosphazene)-based ionic conductors that show good ionic conductivity at room temperature. However, the ionic conductivities are still too low to meet the power density requirements (&gt;100 W kg.sup.-1 sustained power) for high density, rechargeable battery applications.
Because SPEs, such as those based on poly(ethylene oxide) and polyphosphazene, are flexible, maintenance of intimate contact with the solid anode and cathode is less of a problem than with rigid solid electrolytes (e.g., Li-conducting glasses). However, the extent to which contact can be maintained depends on the negative (Li) and positive (e.g., TiS.sub.2, V.sub.6 O.sub.13) electrodes on charging and discharging.
These problems would be greatly alleviated if it were possible to use relatively thick (&gt;500.mu.) SPE films, rather than films of &lt;100.mu. as currently used. The thin films that are now used are dictated by the low lithium ion conductivities of existing SPEs (See, for example, D. F. Shriver et al., Solid State Ionics 5, 83 (1981) and Chem. Eng. News (May 20, 1985) p. 42). Therefore, a principal goal in developing SPE batteries is to increase the cation conductivity. This can be done only by providing new polymer systems that have the necessary structural properties to ensure high and stable cation conductivities under the conditions of interest.
Two factors are critical to the transport of ions in polymer electrolytes: (1) liquid-like (amorphous) character of the polymer and (2) sites in the polymer that loosely bind with the ion to permit diffusion. Thus, having "floppy" polyether pendant groups on the polyphosphazene elastomer greatly reduces the glass transition temperature (T.sub.g) of the polymer. Consequently, when complexed with salts, this polymer shows substantially higher room temperature conductivity than the corresponding PEO complexes. However, the ionic conductance exhibited by the polyphosphazene electrolyte at room temperature is still too low for application in batteries. In addition, these polyphosphazene based electrolytes do not form good uniform flexible films.
The present invention is directed to overcoming one or more of the problems as set forth above.